Browsing by Subject "FDTD"

Now showing 1 - 13 of 13

Open Access
AIBIIICVI 2 (A = Cu, Ag; B = Ga, In; C = S, Se, Te) based photonic crystal superlattices: optical properties
(Wiley-VCH Verlag, 2017) Simsek S.; Palaz S.; Akhundov, C.; Mamedov, A. M.; Özbay, Ekmel
In this study, we present an investigation of the optical properties and band structures for the photonic structures based on AIBIIICVI 2 with a Fibonacci sequence that can act as a multi-wavelength birefringent filter. The filtering wavelengths are analyzed by the indices concerning the quasi-periodicity of a Fibonacci sequence and the average lattice parameter. The transmittances of filtering wavelengths can be tuned by varying structure parameters such as the lengths of poled domains, filling factor, and dispersion relation. In our simulation, we employed the finite-difference time domain (FDTD) technique, which implies a solution from Maxwell equation.
Open Access
Developing a transducer based on localized surface plasmon resonance (LSPR) of gold nanostructures for nanobiosensor applications
(Trans Tech Publications, 2013) Turhan, Adil Burak; Ataman, D.; Çakmakyapan, S.; Mutlu, M.; Özbay, Ekmel; Vlachos, D. S.; Hristoforou, E.
In this work, we report the nanofabrication, optical characterization, and electromagnetic modeling of various nanostructure arrays for localized surface plasmon resonance (LSPR) based biosensing studies. Comparison of the experimental results and simulation outputs of various nanostructure arrays was made and a good correspondence was achieved.
Open Access
An efficient and accurate technique for the incident-wave excitations in the FDTD method
(Institute of Electrical and Electronics Engineers, 1998-06) Oğuz, U.; Gürel, Levent; Arıkan, Orhan
An efficient technique to improve the accuracy of the finite-difference time-domain (FDTD) solutions employing incident-wave excitations is developed. In the separate-field formulation of the FDTD method, any incident wave may be efficiently introduced to the three-dimensional (3-D) computational domain by interpolating from a one-dimensional (1-D) incident-field array (IFA), which is a 1-D FDTD grid simulating the propagation of the incident wave. By considering the FDTD computational domain as a sampled system and the interpolation operation as a decimation process, signal-processing techniques are used to identify and ameliorate the errors due to aliasing. The reduction in the error is demonstrated for various cases. This technique can be used for the excitation of the FDTD grid by any incident wave. A fast technique is used to extract the amplitude and the phase of a sampled sinusoidal signal.
Open Access
Fibonacci sequences quasiperiodic A5B6C7 ferroelectric based photonic crystal: FDTD analysis
(Taylor and Francis Ltd., 2017) Simsek S.; Palaz S.; Mamedov, A. M.; Özbay, Ekmel
In this study, we present an investigation of the optical properties and band structures for the conventional and Fibonacci photonic crystals (PCs) based on some A5B6C7 ferroelectrics (SbSBr and BiTeCl). Here, we use one dimensional SbSBr and BiTeCl based layers in air background. We have theoretically calculated the photonic band structure and transmission spectra of SbSBr and BiTeCl based PC superlattices. The position of minima in the transmission spectrum correlates with the gaps obtained in the calculation. The intensity of the transmission depths is more intense in the case of higher refractive index contrast between the layers. In our simulation, we employed the finite-difference time domain technique and the plane wave expansion method, which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives.
Open Access
Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures
(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.
We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Localized plasmon-coupled semiconductor nanocrystal emitters for innovative device applications
(2007) Soğancı, İbrahim Murat
Quantum confinement allows for the development of novel luminescent materials such as colloidal semiconductor quantum dots for a variety of photonic applications spanning from biomedical labeling to white light generation. However, such device applications require efficient photoluminescence. To this end, in this thesis we investigate the spontaneous emission characteristics of semiconductor nanocrystal emitters under different conditions and their enhancement and controlled modification via plasmonic resonance coupling, placing metallic nanoparticles in their proximity, for innovative device applications. We first present our theoretical and experimental work on the optical characterization of nanocyrstals (e.g., CdSe, CdS, and CdSe/ZnS) including absorption/photoluminescence, time-resolved luminescence, and excitation spectra measurements. Here we demonstrate very strong electromodulation (up to 90%) of photoluminescence and absorption of such nanocrystals (nanodots and nanorods) for optical modulator applications. Second, we present our electromagnetic modeling on the optical response of metal nanoparticles using finite-difference-time-domain method. For the first time, using localized plasmons of metal nanoisland films (nano-silver) carefully spectrally and spatially tuned for optimal coupling conditions, we report very significant controlled modifications of nanocrystal emission including the peak emission wavelength shift (by 14nm), emission linewidth reduction (by 10nm with 22% FWHM reduction), photoluminescence intensity enhancement (15.1- and 21.6-fold compared to the control groups of the same nanocrystals with no plasmonic coupling and those with identical nano-silver but no dielectric spacer in the case of non-radiative energy transfer, respectively), and selectable peaking of surface-state emission at desired wavelengths. Such localized plasmonic engineering of nanocrystal emitters opens new possibilities for our lightemitting and photovoltaic devices.
Open Access
Novel design-based complex nanostructures in hybrid core-shell architectures for high-efficiency light generation
(2010) Özel, İlkem Özge
Recent developments in nanoscience and nanotechnology have given rise to the discovery of hybrid nanostructured multi-component materials that serve several tasks all at once. A very important and rapidly growing field of these materials is the development of highly efficient fluorophores to meet the urgent demand of low-energy consuming, high-quality light emitters for future solid-state lighting applications. Such hybrid nanomaterials are entailed to exhibit extraordinary optoelectronic properties compared to the bulk case of their single components such as enhanced quantum efficiency, tunable multi-color emission, and reduction of multiple processing steps. Herein, to address these requirements, we propose and demonstrate novel design-based complex nanomaterials in hybrid multi-shell architectures for high-efficiency light generation. These requirements are made possible by using the concept of hybrid core-shell-… nanostructures comprising at least two units, including hybrid metalcore/dielectric-shell nanoparticles furnished with an outer shell of semiconductor nanocrystals for enhanced emission and different conjugated polymers forming a single multi-polymer nanoparticle and emitting simultaneously at different wavelengths. In the first part of this thesis, we developed and demonstrated Au-silica core/shell nanoparticles that successfully assemble CdTe nanocrystals right on their silica shells for enhanced plasmonexciton interactions, while solving the common problems of lacking control in dielectric spacing and limited film thickness typically encountered in such plasmon-coupled nanocrystals. Here we present the synthesis and characterization results of this new set of multi-shell decorated nanoparticle composites with a tunable dielectric spacing thickness of silica shell precisely controlled by synthesis to optimize plasmon-exciton interactions for enhanced emission. Experimental data obtained from steady-state and time-resolved photoluminescence measurements together with extensive computational analysis clearly verify the strong plasmon-exciton interactions in these designbased multi-shell nanocomposites. In the second part, we construct bi-polymer nanoparticle systems in various architectures of core/shells, for each of which thorough investigations of the non-radiative energy transfer mechanisms are made. Here we present the synthesis and characterization results of these core/shell bi-polymer nanoassemblies. The flexibility of designing such bipolymer nanostructures allows for the optimization of maximum energy transfer efficiency. This concept of complex hybrid nanostructures for high-efficiency light generation opens up new paths for optoelectronic devices and nanophotonics applications including those in solid-state lighting.
Open Access
Novel optical antennas inspired by metamaterial architectures
(2011) Kılıç, Veli Tayfun
The spatial resolution of conventional optical systems is commonly constrained by the diffraction limit. This is a fundamental problem important for various high-tech applications including density limitation in data storage devices (CD, DVD, and Blue-ray discs), crosstalk in detectors, and blurred images in microscopy. To overcome this limit, different types of optical antennas have been investigated to date. However, these antennas either do not exhibit a maximum level of field intensity enhancement that can be achieved via field localization using plasmons or they have large field intensity enhancement at the cost of complicated three-dimensional architectures or very sharp tips, which are hard to fabricate. In this thesis, to address this problem, we investigate a new class of planar optical antennas inspired by metamaterial architectures including E-shape and comb shape. We found that the field intensity enhancements inside the gap regions of such comb-shaped nanoantennas were significantly increased compared to the single or array of dipoles, despite operating across an electrical length significantly reduced with respect to their resonance wavelength. We also showed that the field intensity localization of a single dipole nanoantenna can be at least doubled using single ring resonator with the same gap size by decreasing field radiations from end points and obtaining continuous current flow. These results indicate that comb-shaped planar nanoantennas hold great promise for strong field localization.
Open Access
Novel volumetric plasmonic resonator architectures for enhanced absorption in thin-film organic solar cells
(2010) Sefünç, Mustafa Akın
There has been a growing interest in decreasing the cost and/or increasing the efficiency of clean renewable energy resources including those of photovoltaic approaches for conversion of sunlight into electricity. Today, although photovoltaics is considered a potential candidate in diversification of energy sources, the cost of photovoltaic systems remains yet to be reduced by several factors to compete with fossil fuel based energy production. To this end, new generation solar cells are designed to feature very thin layers of active (absorbing) materials in the order of tens of nanometers. Though this approach may possibly decrease the cost of solar cells, these ultra-thin absorbing layers suffer from undesirably low optical absorption of incident photons. Recently revolutionary efforts on increasing light trapping using nanopatterned metal layers in the active photovoltaic material via surface plasmon excitations have been demonstrated, which attracted interest of the academic community as well as the industry. In these prior studies, plasmonic structures, placed either on the top or at the bottom of absorbing layers, have been investigated to enhance the absorption in the active material. However, all these previous efforts were based only on using a single layer of plasmonic structures. In this thesis, different than the previous reports of our group and the others, we focus on a new design concept of volumetric plasmonic resonators that relies on the idea of incorporating two (or more) layers of coupled plasmonic structures embedded in the organic solar cells. For proof-of-concept demonstration, here we embody one silver grating on the top of the absorbing layer and another at the bottom of the active layer to couple them with each other such that the resulting field localization is further increased and extended within the volume of the active material. In addition to individual plasmonic resonances of these metallic structures, this allows us to take the advantage of the vertical interaction in the volumetric resonator. Our computational results show that this architecture exhibits a substantial absorption enhancement performance particularly under the transverse-magnetic polarized illumination, while the optical absorption is maintained at a similar level as the top grating alone under the transverseelectric polarized illumination. As a result, the optical absorption in the active layer is enhanced up to ~67%, surpassing the improvement limit of individual gratings, when the total film thickness is kept fixed. This volumetric interaction contributes to further enhancement of optical absorption in the active layer, beyond the limited photon absorption in non-metallic (bare) organic solar cell.
Open Access
Physics and applications of defect structures in photonic crystals
(SPIE, 2003) Özbay, Ekmel; Güven, Kaan; Bayındır, Mehmet
Photonic crystals are three dimensional periodic structures having the property of reflecting the electromagnetic (EM) waves in all dimensions, for a certain range of frequencies. Defects or cavities around the same geometry can also be built by means of adding or removing material. The electrical fields in such cavities are usually enhanced, and by placing active devices in such cavities, one can make the device benefit from the wavelength selectivity and the large enhancement of the resonant EM field within the cavity. By using coupled periodic defects, we have experimentally observed a new type of waveguiding in a photonic crystal. A complete transmission was achieved throughout the entire waveguiding band. The transmission, phase, and delay time characteristics of the various coupled-cavity structures were measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting and switching of electromagnetic waves in waveguide ports, and Mach-Xender interferometer effect in such structures. The corresponding field patterns and the transmission spectra were obtained from the finite-difference-time-domain (FDTD) simulations. We developed a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Plasmonics from metal nanoparticles for solar cell applications
(2013) Günendi, Mehmet Can
In today’s economy, need for development in energy is essential. Solar energy is safe, and at the same time is one of the cleanest, cheapest choices of energy alternative to fossil fuels. In this perspective, using the sun light effectively is in fundamental importance. One of the problems, because of the indirect band gap of the material Si, is small energy conversion ratios of various solar cell structures and limited absorption of red light. Because of the material properties, Si cells cannot absorb red light, which contributes great amount of the sun light. One of the recent developed techniques to use red light is using metal nanoparticles (MNP) embedded in a semiconductor medium as sub-wavelength antennas or MNP scatterers, hence increasing the effective path length of light in the cell. Absorption and scattering are mostly in plasmon resonances. Shifting the plasmon resonance peaks is possible by changing various parameters of the system like the size of the MNPs. In this work, Finite-Difference Time-Domain (FDTD) method is used to analyze various systems worked. Mainly the MEEP package, developed at MIT, is used to simulate systems and other codes, related to analytical work, have also used to compare results. The plasmon resonances of various sizes of Ag MNPs embedded in different mediums at different positions are analyzed. Critical parameters like particle size, shape, dielectric medium, film thickness are discussed for improved solar cell applications.
Open Access
Radiation properties of sources inside photonic crystals
(2003) Bulu, İrfan
The control of spontaneous emission is an important problem both in basic and applied physics. Two main problems arise in the control of emission: enhancement or suppression and angular confinement of radiation. In this work we studied the properties of emission of radiation from a localized microwave source embedded inside a photonic crystal. We showed that by using a photonic crystal it is possible to enhance the emitted power. We achieved up to 22 times enhancement of power at the band edge of the photonic crystal. We also studied the properties of emission of radiation from a source embedded inside a single defect structure and embedded inside a coupled defect structure. Enhanced emission for single defect and coupled defect structures was also observed. Moreover, angular distribution of power from a localized microwave source embedded inside a photonic crystal was studied. Angular confinement was achieved near the band edge of the photonic crystal. Half power beam widths as small as 6 degrees were obtained. This is the smallest half power beam width in the literature obtained by using photonic crystals. We also investigated frequency and size dependence of the angular distribution. We observed that the angular confinement strongly depends on frequency and on the size of the photonic crystal. In fact, we showed that angular confinement could be obtained just at the band edge frequency. In conclusion, our work showed that the problem of controlling the spontaneous emission could be solved at once by using photonic crystals.
Open Access
Selective plasmonic control of excitons and their non-radiative energy transfer in colloidal semiconductor quantum dot solids
(2009) Özel, Tuncay
To date extensive research has proved that semiconductors and metals exhibit extraordinary optical properties in nano-dimensions compared to their bulk counterparts. For example, an interesting effect is observed in metal nanostructures/nanoparticles (NPs) that we form to obtain localized plasmons, with their optical response highly tuneable using the size effect. Another field of interest at the nanoscale is the investigation of light generation and harvesting using colloidal semiconductor quantum dot nanocrystals (NCs) that we synthesize in few nanometers, with their emission and absorption excitonic peaks conveniently tuneable using the size effect. In this thesis, we proposed and demonstrated the first accounts of selectively plasmonically-controlled colloidal quantum dot emitters assembled in innovative architectures, with a control achieved either through spatial selection or spectral selection. In the first set of designs, we developed for the first time plasmonic NC-composites that rely on spatially-selected plasmon-coupled CdTe NC-monolayers interspaced with respect to Au NP-monolayers in a repeating three-dimensional layer-by-layer architecture. In these bottom-up designs of hybrid nanocomposites, the photoluminescence kinetics is strongly modified and a record quantum efficiency of 30% is achieved for such CdTe NC solids. In the second set of designs, we showed the first spectrally-selected plasmon-coupling of surfaceemitting CdS NCs using optimized Ag NP deposits. This architecture allowed for the surface-state emission to be selectively enhanced while the interband emission is simultaneously suppressed in the same plasmon-coupled NCs, leading to the strongest surface-state emission from such CdS NCs reported with respect to their interband emission (with a >12-fold enhancement). Yet another important proximity phenomenon effective among quantum dot emitters is the Förster-type non-radiative resonance energy transfer (ET), in which excitonic excitation energy of the donor-NCs is non-radiatively transferred to the acceptor-NCs via dipole-dipole coupling. In the third set of our designs, we combined two fundamental proximity mechanisms of plasmon coupling and non-radiative energy transfer in the same NC solids. In plasmonic ET, we reported for the first time selectively plasmon-coupling of NC-acceptors and then that of NC-donors in the ET pair, both of which result in substantial enhancement of the acceptor emission with respect to ET with no plasmon coupling (with a maximum of 2-fold enhancement) as verified by their steadystate and time-resolved photoluminescence. This concept of spectrally/spatiallyselective plasmon coupling in quantum dots paves a new path for devices and sensors in nanophotonics.